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What kind of witchcraft is this?

P

Plimpington2

Super Member
I am about to dig into a very crusty Model 2000 and when I pulled up the service manual, it appears by all measures that the line-level "Aux" input also feeds into the head amp. That can't be right! There must be something going on here that I am not getting. Aux is input #6 on the switch, and the switch seems to send everything into the same stage. What gives? Below is the schematic for one channel for everything proximal to the volume pot. This channel gets 2 of the wafers on the switch (S1b and S1d) and I gather that S1d (at the bottom) will select the signal to enter the feedback loop when on "phono" of "Tape HD" (position 1 and 2), but somehow "undo" the compensation and extra gain when choosing a line level input? Is that how this thing works?

Schematic of input section .png
 
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Plimpington2 said:
it appears by all measures that the line-level "Aux" input also feeds into the head amp. That can't be right! There must be something going on here that I am not getting.
Click to expand...
Several/many of the lesser model Sansui's had this feature, it saved a couple of transistors per channel, and is quite normal. They switched out the RIAA components when the gain it provided wasn't being used for the Phono input. (I think in some models it became the tone control amplifier too!).

Welcome to the World of early Sansui's.
 
Hyperion said:
Several/many of the lesser model Sansui's had this feature, it saved a couple of transistors per channel, and is quite normal. They switched out the RIAA components when the gain it provided wasn't being used for the Phono input. (I think in some models it became the tone control amplifier too!).

Welcome to the World of early Sansui's.
Click to expand...
Well, I have it up and running now. It absolutely puts me in mind of how the AU-505 sounded, which I rehabbed and then stupidly sold last year. Like the 505, it has this weird capacity to remain utterly composed even as you advance the volume past stupid levels. I’m definitely not letting this one go. Now I just gotta work out how to jumper the input selector switch so the tuner display remains always on.
 
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